OK, I know it isn't kosher, but what is the shiur? And do we hold "chetzi shiur assur min haTorah"?
For an application, what if you get a cut while hiking in the woods; is it assur to clean the wound with your mouth? (am I absolutely disgusting to even think to do this? It seems quite natural...)

What about if you bite your tongue or cheek; are you required to soak up the blood with a napkin until it stops bleeding? Spit out any blood you might taste?

I also received an answer from a doctor, who said that saliva was actually very bad for cleaning a wound, as it could introduce all sorts of bacteria. So I will certainly stop that practice!
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JeremyJun 1 '10 at 13:23

I'm pretty sure it is kosher. Why do you assume the other way?
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Double AA♦Dec 14 '11 at 23:41

Human blood is prohibited because of Maris Ayin, it is not even strictly speaking a "Rabbinic prohibition". As such there is, I believe, no concept of tam k'ikkar (the taste is like the forbidden substance). As such while it is prohibited to eat bread which has blood from one's gums on it, such blood would not impart a forbidden taste to utensils. (Shulchan Aruch, Yoreh Deah 66:10)"

As mentioned, blood from one's gums isn't a problem unless it leaves one's mouth in which case it looks like one is eating blood and is prohibited due to ma'aris ayin.

Since tam/taste is not the issue it is possible that it is not a problem to suck blood from a wound when once spits it out immediately. Obviously if there is a real sakana it is permitted. I'll try to find sources that deal with it more directly.

It is interesting to note the discussion as to the kosher status of human flesh. The Torah Ladaat has an article on this on parashat shemini. He brings the Rambam (perek 2 of maachalot assurot, halacha 3):

The Ritva and the Ran (Ketuvot 60a) agree with the above ruling as does the Rama (יו''ד ס' עט:א). Raavad, Tosafot, Rosh, Ramban, Rashbah disagree, that there isn't an עשה prohibition מדאורייתא (unsourced in Torah Ladaat).

The Torah Ladaat then describes the subsequent question of human derivatives e.g. blood and breast milk. He says that even according to those who permit it there is a mitzvah of 'prisha' and it is considered an abomination to eat it. He then sites the Pri Hadash who disagrees in the aspect of prisha, but I am unsure whether this refers only to breast milk, or human blood as well (didn't look up that source).